Transmission lines may generally be designed to carry, for example, alternating current or radio frequency signals. One of the most common types of transmission line is a coaxial cable. Transmission lines are commonly used in mobile devices (e.g., phones) to transmit a signal from a controller circuit to one or more antenna circuits in a mobile telephone. As a result, the signal transmission line may be configured to transmit signals with a wide range of frequencies. For example, signal transmission line can be configured to carry signals for a Bluetooth antenna, a Wi-Fi antenna, or a mobile communications antenna operating at various frequencies. While robust, coaxial cables can be too bulky for use in mobile devices. Another type of signal transmission line is a stripline signal transmission line. In the stripline structure, a signal line can be sandwiched between an upper and a lower grounding conductor with an insulating material disposed between the conductors and the signal line. The insulating material of a substrate can form the dielectric. The width of the signal line, the thickness of the substrate, and the relative permittivity of the substrate can determine the characteristic impedance of the stripline structure. There remains a need for improvements to signal transmission lines, especially for use in mobile devices.